Single axis light pipe for homogenizing slow axis of illumination systems bases on laser diodes

ABSTRACT

Apparatus for thermally processing a semiconductor wafer includes an array of semiconductor laser emitters arranged in plural parallel rows extending along a slow axis, plural respective cylindrical lenses overlying respective ones of the rows of laser emitters for collimating light from the respective rows along a fast axis generally perpendicular to the slow axis, a homogenizing light pipe having an input face at a first end for receiving light from the plural cylindrical lenses and an output face at an opposite end, the light pipe comprising a pair of reflective walls extending between the input and output faces and separated from one another along the direction of the slow axis, and scanning apparatus for scanning light emitted from the homogenizing light pipe across the wafer in a scanning direction parallel to the fast axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/627,238, filed Nov. 12, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to thermal processing of semiconductorsubstrates. In particular, the invention relates to laser thermalprocessing of semiconductor substrates.

2. Background Art

Thermal processing is required in the fabrication of silicon and othersemiconductor integrated circuits formed in silicon wafers or othersubstrates such as glass panels for displays. The required temperaturesmay range from relatively low temperatures of less than 250° C. togreater than 1000°, 1200°, or even 1400° C. and may be used for avariety of processes such as dopant implant annealing, crystallization,oxidation, nitridation, silicidation, and chemical vapor deposition aswell as others.

For the very shallow circuit features required for advanced integratedcircuits, it is greatly desired to reduce the total thermal budget inachieving the required thermal processing. The thermal budget may beconsidered as the total time at high temperatures necessary to achievethe desired processing temperature. The time that the wafer needs tostay at the highest temperature can be very short.

Rapid thermal processing (RTP) uses radiant lamps which can be veryquickly turned on and off to heat only the wafer and not the rest of thechamber. Pulsed laser annealing using very short (about 20 ns) laserpulses is effective at heating only the surface layer and not theunderlying wafer, thus allowing very short ramp up and ramp down rates.

A more recently developed approach in various forms, sometimes calledthermal flux laser annealing or dynamic surface annealing (DSA), isdescribed by Jennings et al. in PCT/2003/00196966 based upon U.S. patentapplication Ser. No. 10/325,497, filed Dec. 18, 2002 and incorporatedherein by reference in its entirety. Markle describes a different formin U.S. Pat. No. 6,531,681 and Talwar yet a further version in U.S. Pat.No. 6,747,245.

The Jennings and Markle versions use CW diode lasers to produce veryintense beams of light that strikes the wafer as a thin long line ofradiation. The line is then scanned over the surface of the wafer in adirection perpendicular to the long dimension of the line beam.

SUMMARY OF THE INVENTION

Apparatus for thermally processing a semiconductor wafer includes anarray of semiconductor laser emitters arranged in plural parallel rowsextending along a slow axis, plural respective cylindrical lensesoverlying respective ones of the rows of laser emitters for collimatinglight from the respective rows along a fast axis generally perpendicularto the slow axis, a homogenizing light pipe having an input face at afirst end for receiving light from the plural cylindrical lenses and anoutput face at an opposite end, the light pipe comprising a pair ofreflective walls extending between the input and output faces andseparated from one another along the direction of the slow axis, andscanning apparatus for scanning light emitted from the homogenizinglight pipe across the wafer in a scanning direction parallel to the fastaxis. Lenses focus light derived from the output face of the light pipeinto a line of light on the wafer, the line of light having an elongatedimension along the slow axis and a narrow dimension along the fastaxis, wherein the scanning apparatus scans the line of light across thewafer along the fast axis. The reflective walls of the light pipe aresufficiently close to one another to facilitate multiple reflectionsacross the slow axis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an orthographic representation of a thermal flux laserannealing apparatus employed in the present invention.

FIGS. 2 and 3 are orthographic views from different perspectives ofoptical components of the apparatus of FIG. 1.

FIG. 4 is an end plan view of a portion of a semiconductor laser arrayin the apparatus of FIG. 1.

FIG. 5 is an orthographic view of a homogenizing light pipe for theapparatus of FIG. 1.

FIG. 6 is a perspective view of the light pipe of FIG. 5 and of the lensassemblies at its input and output faces.

FIG. 7 is a top view of the light pipe of FIG. 6 along the fast axis.

FIG. 8 is a side view of the light pipe of FIG. 6 along the slow axis.

FIG. 9 is an orthographic view of an embodiment of the light pipe ofFIG. 5 formed as a truncated wedge having decreasing cross-sectionalarea along the optical axis.

FIG. 10 is an orthographic view of an embodiment of the light pipe ofFIG. 5 formed as a truncated wedge having increasing cross-sectionalarea along the optical axis.

FIG. 11 is diagram of multiple reflections inside the light pipe of FIG.10, illustrating the effects of a beam diverging lens at the input ofthe light pipe.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the apparatus described in the above-referencedapplication by Jennings et al. is illustrated in the schematicorthographic representation of FIG. 1. A gantry structure 10 fortwo-dimensional scanning includes a pair of fixed parallel rails 12, 14.Two parallel gantry beams 16, 18 are fixed together a set distance apartand supported on the fixed rails 12, 14 and are controlled by anunillustrated motor and drive mechanism to slide on rollers or ballbearings together along the fixed rails 12, 14. A beam source 20 isslidably supported on the gantry beams 16, 18, and may be suspendedbelow the beams 16, 18 which are controlled by unillustrated motors anddrive mechanisms to slide along them. A silicon wafer 22 or othersubstrate is stationarily supported below the gantry structure 10. Thebeam source 20 includes a laser light source and optics to produce adownwardly directed fan-shaped beam 24 that strikes the wafer 22 as aline beam 26 extending generally parallel to the fixed rails 12, 14, inwhat is conveniently called the slow direction. Although not illustratedhere, the gantry structure further includes a Z-axis stage for movingthe laser light source and optics in a direction generally parallel tothe fan-shaped beam 24 to thereby controllably vary the distance betweenthe beam source 20 and the wafer 22 and thus control the focusing of theline beam 26 on the wafer 22. Exemplary dimensions of the line beam 26include a length of 1 cm and a width of 66 microns with an exemplarypower density of 220 kW/cm². Alternatively, the beam source andassociated optics may be stationary while the wafer is supported on astage which scans it in two dimensions.

In typical operation, the gantry beams 16, 18 are set at a particularposition along the fixed rails 12, 14 and the beam source 20 is moved ata uniform speed along the gantry beams 16, 18 to scan the line beam 26perpendicularly to its long dimension in a direction conveniently calledthe fast direction. The line beam 26 is thereby scanned from one side ofthe wafer 22 to the other to irradiate a 1 cm swath of the wafer 22. Theline beam 26 is narrow enough and the scanning speed in the fastdirection fast enough that a particular area of the wafer is onlymomentarily exposed to the optical radiation of the line beam 26 but theintensity at the peak of the line beam is enough to heat the surfaceregion to very high temperatures. However, the deeper portions of thewafer 22 are not significantly heated and further act as a heat sink toquickly cool the surface region. Once the fast scan has been completed,the gantry beams 16, 18 are moved along the fixed rails 12, 14 to a newposition such that the line beam 26 is moved along its long dimensionextending along the slow axis. The fast scanning is then performed toirradiate a neighboring swath of the wafer 22. The alternating fast andslow scanning are repeated, perhaps in a serpentine path of the beamsource 20, until the entire wafer 22 has been thermally processed.

The optics beam source 20 includes an array of lasers. An example isorthographically illustrated in FIGS. 2 and 3, in which laser radiationat about 810 nm is produced in an optical system 30 from two laser barstacks 32, one of which is illustrated in end plan view in FIG. 4. Eachlaser bar stack 32 includes 14 parallel bars 34, generally correspondingto a vertical p-n junction in a GaAs semiconductor structure, extendinglaterally about 1 cm and separated by about 0.9 mm. Typically, watercooling layers are disposed between the bars 34. In each bar 34 areformed 49 emitters 36, each constituting a separate GaAs laser emittingrespective beams having different divergence angles in orthogonaldirections. The illustrated bars 34 are positioned with their longdimension extending over multiple emitters 36 and aligned along the slowaxis and their short dimension corresponding to the less than 1-micronp-n depletion layer aligned along the fast axis. The small source sizealong the fast axis allows effective collimation along the fast axis.The divergence angle is large along the fast axis and relatively smallalong the slow axis.

Returning to FIGS. 2 and 3 two arrays of cylindrical lenslets 40 arepositioned along the laser bars 34 to collimate the laser light in anarrow beam along the fast axis. They may be bonded with adhesive on thelaser stacks 32 and aligned with the bars 34 to extend over the emittingareas 36.

The optics beam source 20 can further include conventional opticalelements. Such conventional optical elements can include an interleaverand a polarization multiplexer, although the selection by the skilledworker of such elements is not limited to such an example. In theexample of FIGS. 2 and 3, the two sets of beams from the two bar stacks32 are input to an interleaver 42, which has a multiple beam splittertype of structure and having specified coatings on two internal diagonalfaces, e.g., reflective parallel bands, to selectively reflect andtransmit light. Such interleavers are commercially available fromResearch Electro Optics (REO). In the interleaver 42, patterned metallicreflector bands are formed in angled surfaces for each set of beams fromthe two bar stacks 32 such that beams from bars 34 on one side of thestack 32 are alternatively reflected or transmitted and therebyinterleaved with beams from bars 34 on the other side of the stack 32which undergo corresponding selective transmission/reflection, therebyfilling in the otherwise spaced radiation profile from the separatedemitters 36.

A first set of interleaved beams is passed through a quarter-wave plate48 to rotate its polarization relative to that of the second set ofinterleaved beams. Both sets of interleaved beams are input to apolarization multiplexer (PMUX) 52 having a structure of a doublepolarization beam splitter. Such a PMUX is commercially available fromResearch Electro Optics. First and second diagonal interface layers 54,56 cause the two sets of interleaved beams to be reflected along acommon axis from their front faces. The first interface 54 is typicallyimplemented as a dielectric interference filter designed as a hardreflector (HR) while the second interface 56 is implemented as adielectric interference filter designed as a polarization beam splitter(PBS) at the laser wavelength. As a result, the first set of interleavedbeams reflected from the first interface layer 54 strikes the back ofthe second interface layer 56. Because of the polarization rotationintroduced by the quarter-wave plate 48, the first set of interleavedbeams passes through the second interface layer 56. The intensity of asource beam 58 output by the PMUX 52 is doubled from that of the eitherof the two sets of interleaved beams.

Although shown separated in the drawings, the interleaver 42, thequarter-wave plate 48, and the PMUX 52 and its interfaces 54, 56, aswell as additional filters that may be attached to input and outputfaces are typically joined together by a plastic encapsulant, such as aUV curable epoxy, to provide a rigid optical system. An importantinterface is the plastic bonding of the lenslets 40 to the laser stacks32, on which they must be aligned to the bars 34. The source beam 58 ispassed through a set of cylindrical lenses 62, 64, 66 to focus thesource beam 58 along the slow axis.

A one-dimensional light pipe 70 homogenizes the source beam along theslow axis. The source beam, focused by the cylindrical lenses 62, 64,66, enters the light pipe 70 with a finite convergence angle along theslow axis but substantially collimated along the fast axis. The lightpipe 70, more clearly illustrated in the orthographic view of FIG. 5,acts as a beam homogenizer to reduce the beam structure along the slowaxis introduced by the multiple emitters 36 in the bar stack 32 spacedapart on the slow axis. The light pipe 70 may be implemented as arectangular slab 72 of optical glass having a sufficiently high index ofrefraction to produce total internal reflection. It has a shortdimension along the slow axis and a longer dimension along the fastaxis. The slab 72 extends a substantial distance along an axis 74 of thesource beam 58 converging along the slow axis on an input face 76. Thesource beam 58 is internally reflected several times from the top andbottom surfaces of the slab 72, thereby removing much of the texturingalong the slow axis and homogenizing the beam along the slow axis whenit exits on an output face 78. The source beam 58, however, is alreadywell collimated along the fast axis (by the cylindrical lensets 40) andthe slab 72 is wide enough that the source beam 58 is not internallyreflected on the side surfaces of the slab 72 but maintains itscollimation along the fast axis. The light pipe 70 may be tapered alongits axial direction to control the entrance and exit apertures and beamconvergence and divergence. The one-dimensional light pipe canalternatively be implemented as two parallel reflective surfacescorresponding generally to the upper and lower faces of the slab 72 withthe source beam passing between them.

The source beam output by the light pipe 70 is generally uniform. Asfurther illustrated in the schematic view of FIG. 6, further anamorphiclens set or optics 80 that includes cylindrical lenses 81, 82, expandsthe output beam in the slow axis, and further includes a generallyspherical lens 83 to project the desired line beam 26 on the wafer 22.The anamorphic optics 80 shape the source beam in two dimensions toproduce a narrow line beam of limited length. In the direction of thefast axis, the output optics have an infinite conjugate for the sourceat the output of the light pipe (although systems may be designed with afinite source conjugate) and a finite conjugate at the image plane ofthe wafer 22 while, in the direction of the slow axis, the output opticshas a finite conjugate at the source at the output of the light pipe 70and a finite conjugate at the image plane. Further, in the direction ofthe slow axis, the nonuniform radiation from the multiple laser diodesof the laser bars is homogenized by the light pipe 70. The ability ofthe light pipe 70 to homogenize strongly depends on the number of timesthe light is reflected traversing the light pipe 70. This number isdetermined by the length of the light pipe 70, the direction of thetaper if any, the size of the entrance and exit apertures as well as thelaunch angle into the light pipe 70. The output optics 80 focus thesource beam into the line beam of desired dimensions on the surface ofthe wafer 22.

FIGS. 7 and 8 are perpendicularly arranged side views along the fast andslow axes respectively showing the light pipe 70 and some associatedoptics. In the direction of the fast axis, the beam from the lasers bars32 is well collimated and not affected by the light pipe 70 oranamorphic optics. On the other hand, in the direction of the slow axis,the input anamorphic optics 62, 64, 66 condense and converge the beaminto the input end of the light pipe 70. The beam exits the light pipe70 with substantially uniform intensity along the slow axis but with asubstantial divergence. The output anamorphic optics 80 expand andcollimate the output beam along the slow axis.

The light pipe 70 described above has a uniform rectangular crosssection along the optical axis 74. However, tapered profiles with crosssections tapering along the optical axis 74 may be advantageously usedin combination with the subsequent optics. In particular, a taperedlight pipe increases the number of reflections occurring over a fixedlength of the light pipe. A dielectric light pipe 90 illustratedorthographically in FIG. 9 is formed from a truncated wedge 92 ofoptical glass with a uniformly decreasing rectangular cross sectionalong the optical axis 74 from an input face 94 to an output face 96.That is, the aspect ratio of the light pipe 90 is continuallyincreasing, for example, from 5:1 to 10:1, producing a ratio of aspectratios, for example, of at least 2. In particular, the dimension alongthe slow axis is decreasing and the dimension along the fast axis may bemaintained constant. The advantage of the narrow output face 96 is thatits numerical aperture (NA) is higher, that is, the output beamdivergence is greater.

A complementary configuration is a dielectric light pipe 100 illustratedorthographically in FIG. 10 formed of a truncated wedge 102 of opticalglass with a uniformly increasing rectangular cross section along theoptical axis from an input face 104 to an output face 106 so that theaspect ratio of the wedge 102 is continually decreasing, for example, byratios reverse to those of the previous embodiment. In particular, thedimension along the slow axis is increasing and the dimension along thefast axis may be maintained constant. This configuration has theadvantage that the NA of the wide output face 106 is lower and theoutput beam divergence is less. Advantageously a cylindrical lens 108placed near the input face and extending along the long lateraldirection of the light pipe 100 focuses a somewhat collimated input beam112 into a sharply converging beam at the input face 104. As illustratedin the side cross sectional view of FIG. 11, lateral beam components 114at the ends of the slow direction bounce many times near the small endof the tapered light pipe 100 and are gradually brought closer to beparallel to the optical axis. As a result, the output beam has a smallNA and relatively large size along the slow axis.

It is appreciated that the lateral side walls of the dielectric lightpipes 70, 90, 100 do not really participate in the action of the lightpipe such that a single-axis light pipe is obtained in which noreflecting or homogenizing is obtained in along the long lateraldirection of the pipe. Hence, it is not required that those lateralswalls be parallel although such parallel walls ease fabrication.

The one-dimensional light pipe can alternatively be implemented as twoparallel or slightly inclined reflective surfaces correspondinggenerally to the upper and lower faces of the slab 72 or wedges 92, 102with the source beam passing between them. The reflective surfaces canbe formed as free-standing mirrors or as coatings on a transparentmember not providing total internal reflection. It may be possible tocarry out the invention without either the interleaver 42 or thepolarization multiplexer 52 or without both of them.

While the invention has been described in detail by specific referenceto preferred embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

1. A thermal processing system, comprising: a source of laser radiationemitting at a laser wavelength and comprising a plurality of laserdiodes formed in a substrate and emitting along an optical axis fromemitter areas arranged along a first axis; and a single axis light pipehaving reflective walls separated along said first axis with respect tosaid optical axis, wherein said optical axis passes between saidreflective walls.
 2. The system of claim 1, further comprising at leastone anamorphic lens over said emitter areas of said laser diodes andextending along said first axis to substantially collimate radiationfrom said laser diodes along a second axis perpendicular to said firstaxis.
 3. The system of claim 2, wherein said light pipe does not reflectsaid radiation with respect to said second axis.
 4. The system of claim2, wherein said first axis is a slow axis of a line beam scanningapparatus and said second axis is a fast axis thereof.
 5. The system ofclaim 1, wherein said light pipe is tapered along said first axis. 6.The system of claim 5, wherein said light pipe has an expanding taperalong said optical axis from said source.
 7. The system of claim 5,wherein said light pipe has a contracting taper along said optical axisfrom said source.
 8. The system of claim 4 further comprising scanningapparatus for producing relative motion along said fast axis betweensaid optical axis and a workpiece.
 9. The system of claim 1 wherein saidreflective walls are spaced from one another by a sufficiently shortdistance across said first axis to afford multiple reflections acrosssaid first axis of light between said reflective walls from said sourceof laser radiation.
 10. The system of claim 9 further comprising a pairof support walls coupled to said reflective walls, said support wallsseparated from one another across said second axis by a sufficientlylarge distance to prevent reflection across said second axis betweensaid support walls.
 11. The system of claim 10 further comprisingrespective cylindrical lenses axially extending along said first axisand resting on respective rows of the emitter areas for collimatinglight from said laser source along said second axis.
 12. The system ofclaim 9 wherein said light pipe is tapered to have increased spacingbetween said reflective surfaces along said optical axis from an inputface to an output face of said light pipe.
 13. The system of claim 9wherein said light pipe is tapered to have decreased spacing betweensaid reflective surfaces along said optical axis from an input face toan output face of said light pipe.
 14. Apparatus for processing asemiconductor wafer, comprising: an array of semiconductor laseremitters arranged in plural parallel rows extending along a slow axis;plural respective cylindrical lenses overlying respective ones of saidrows of laser emitters for collimating light from the respective rowsalong a fast axis generally perpendicular to said slow axis; ahomogenizing light pipe having an input face at a first end forreceiving light collimated by said plural cylindrical lenses and anoutput face at an opposite end, said light pipe comprising a pair ofreflective surfaces extending between said input and output faces andseparated from one another along the direction of said slow axis; andscanning apparatus for scanning light emitted from said homogenizinglight pipe across the wafer in a scanning direction parallel to saidfast axis.
 15. The apparatus of claim 14 further comprising optics forfocusing light derived from the output face of said light pipe into aline of light on said wafer, said line of light having an elongatedimension along said slow axis and a narrow dimension along said fastaxis, wherein said scanning apparatus scans said line of light acrosssaid wafer along said fast axis.
 16. The apparatus of claim 14 whereinsaid reflective surfaces are sufficiently close to one another tofacilitate multiple reflections across said slow axis.
 17. The apparatusof claim 16 further comprising a pair of support surfaces coupled tosaid pair of reflective surfaces and being separated from one anotheralong the direction of said fast axis by a sufficiently large distanceto prevent multiple reflections between them along said fast axis. 18.The apparatus of claim 14 wherein said reflective surfaces define atruncated wedge shape, said apparatus further comprising a lens at saidinput face of said light pipe for increasing beam divergence along saidslow axis.
 19. The apparatus of claim 18 wherein said truncated wedgeshape has a decreasing cross-sectional area along the direction of lightpropagation.
 20. A method of annealing a workpiece, comprising: emittingplural light beams from an array of semiconductor laser emittersarranged in plural parallel rows extending along a slow axis;collimating, along a fast axis generally perpendicular to said slowaxis, the light beams from each respective row of laser emitters inrespective cylindrical lenses overlying respective ones of said rows oflaser emitters; making multiple reflections across said slow axis in alight pipe of the light from said plural cylindrical lenses between apair of reflective surfaces separated from one another along thedirection of said slow axis to produce a light beam homogenized alongsaid slow axis; focusing said homogenized light beam onto said workpieceas line of light having a broad dimension along said slow axis and anarrow dimension along said fast axis; and scanning said line of lightacross the workpiece in a scanning direction parallel to said fast axis.